1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and a separator in a stacking direction. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. A reactant gas passage acting as a passage for a reactant gas extends through the separator in the stacking direction.
2. Description of the Related Art
A polymer electrolyte fuel cell employs a membrane electrode assembly (MEA), which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. Normally, a predetermined number of power generation cells are stacked together, wherein terminal plates, insulating plates, and end plates are disposed at opposite ends thereof to form a fuel cell stack.
In the fuel cell, in order to ensure that power is generated effectively, a desired humidified state of the electrolyte membrane needs to be maintained. For this  purpose, for example, an external humidification method is known. In this method, a humidification apparatus for humidifying both the fuel gas and an oxygen-containing gas using water is provided. The humidification apparatus is connected to the fuel cell for supplying the humidified fuel and oxygen-containing gases to the fuel cell.
An internal humidification method is also known. In this method, a humidification unit and a fuel cell are formed integrally. For example, Japanese Laid-Open Patent Publication No. 2002-25584 discloses a polymer electrolyte fuel cell as shown in FIG. 11. The fuel cell comprises a membrane electrode assembly 1, an anode side separator 2a, and a cathode side separator 2k. 
The membrane electrode assembly 1 includes an anode 6a and a cathode 6k formed by joining catalyst layers 4a, 4k and diffusion layers 5a, 5k on both surfaces of an electrolyte membrane 3. The membrane electrode assembly 1 includes humidification sections 7 disposed in areas of the electrolyte membrane where the catalyst layers 4a, 4k are not present. Anode gas flow grooves 8a and cathode gas flow grooves 8k, having serpentine patterns for example, are formed in the anode side separator 2a and the cathode side separator 2k, respectively.
The anode gas and the cathode gas flow respectively in a counterflowing manner. Therefore, after the cathode gas has been humidified by water produced in a reaction within the catalyst layer 4k, the water moves from the cathode 6k to the anode 6a via the humidification section 7 on the upper side, due to a concentration gradient of the water vapor. The anode gas is humidified by water moving from the humidification section 7. In the humidification section 7 on the lower side, similarly, water moves from the anode 6a to the cathode 6b due to a concentration gradient of the water vapor. Thus, the cathode gas is humidified before the cathode gas is consumed in the reaction.
In the conventional technique, water vapor is exchanged between the anode gas flowing through the anode gas flow grooves 8a and the cathode gas flowing through the cathode gas flow grooves 8k. Therefore, the electrode surface area that is used for power generation is reduced by the presence of the humidification sections 7, which are provided on upper and lower sides of the anode gas flow grooves 8a and the cathode gas flow grooves 8k. Thus, the output of the fuel cell is lowered, and the overall size of the fuel cell must be made considerably large in order to achieve a sufficient electrode surface area.
Further, since a relatively stable gas flow occurs in each of the continuous anode gas flow grooves 8a and the continuous cathode gas flow grooves 8k, the flow rate of gas that flows near the surface of the humidification section 7 tends to be decreased undesirably. As a result, water vapor permeability per unit area is lowered, and the surface area of the humidification section 7 needs to be made considerably large in order to achieve a desired humidified  state.